Abstract. Interest is growing in using soft sediment as a foundation in eco-engineering projects. Wetland construction in the Dutch lake Markermeer is an example: here, dredging some of the clay-rich lake-bed sediment and using it to construct wetland will soon begin. Natural processes will be utilized during and after construction to accelerate ecosystem development. Knowing that plants can eco-engineer their environment via positive or negative biogeochemical plant–soil feedbacks, we conducted a 6-month greenhouse experiment to identify the key biogeochemical processes in the mud when Phragmites australis is used as an eco-engineering species. We applied inverse biogeochemical modeling to link observed changes in pore water composition to biogeochemical processes. Two months after transplantation we observed reduced plant growth and shriveling and yellowing of foliage. The N : P ratios of the plant tissue were low, and these were affected not by hampered uptake of N but by enhanced uptake of P. Subsequent analyses revealed high Fe concentrations in the leaves and roots. Sulfate concentrations rose drastically in our experiment due to pyrite oxidation; as reduction of sulfate will decouple Fe-P in reducing conditions, we argue that plant-induced iron toxicity hampered plant growth, forming a negative feedback loop, while simultaneously there was a positive feedback loop, as iron toxicity promotes P mobilization as a result of reduced conditions through root death, thereby stimulating plant growth and regeneration. Given these two feedback mechanisms, we propose the use of Fe-tolerant species rather than species that thrive in N-limited conditions. The results presented in this study demonstrate the importance of studying the biogeochemical properties of the situated sediment and the feedback mechanisms between plant and soil prior to finalizing the design of the eco-engineering project.

Background and aims Understanding the potential effects of iron toxicity on plant development is important when constructing new wetland from iron-rich sediment. We aim to study plant species-specific effects of iron toxicity when grown in the iron-rich sediments of lake Markermeer (the Netherlands).Methods Using three sediment sources that varied in total Fe and Fe-P concentrations, we performed a greenhouseexperiment to study the development of three wetland species that differ in their tolerance to iron andutilization capacity of Fe-P: Rumex maritimus, Phragmites australis and Eupatorium cannabinum.ResultsPhragmites australis was the only species that developed an epidermis-damaging iron plaque on itsroots. Plaque formation mainly depended on the Fe(III) and Fe-P concentration of the sediment, whichled to different nutrient imbalances in leaves. All three species showed reduced growth compared to the controlsubstrate, which could not be linked to indirect Fe toxicity. In contrast, direct Fe toxicity following theuptake of Fe could not be excluded as a mechanism potentially explaining our results, and this result warrantsfurther examination in longer-term experiments.Conclusions Our results highlight the importance of considering the Fe and Fe-P availability in sediments,as these properties may constrain plant performance anddelay the development of pioneer ecosystems in wetlandconstruction sites.